The next holder of the lands of the earldom of Pembroke was WILLIAM DE VALENCE (d. 1296), a younger son of Hugh de Lusignan, count of La Marche, by his marriage with Isabel of Angouleme (d. 1246), widow of the English king John; he was born at Valence, near Lusignan. In 1247 William and his brothers Guy and Aymer, crossed over to England at the invitation of their half-brother, Henry III, who arranged a marriage between William and Joan De Munchensi (d. 1307), a granddaughter of William Marshall, 1st earl of Pembroke. He was hated as one of the most prominent of the rapacious foreigners. He quarrelled with Simon De Montfort, refused to comply with the provisions of Oxford, and took refuge in Wolvesey castle in Winchester, where we as besieged and compelled to surrender and leave the country. In 1259 he and Simon De Montfort were formally reconciled in Paris, and in 1261 he was again in England. He fought for Henry at the battle of Lewes, Sussex, and then, after a stay in France, he landed in Pembrokeshire and took part in 1265 in the siege of Gloucester, Gloucestershire, and the battle of Evesham, Worcestershire, on 4 August 1265. After the royalist victory he was restored to his estates and accompanied Prince Edward, afterward Edward I, to Palestine. William de Valence, 1st Earl of Wexford and 1st Earl of Pembroke, born Guillaume de Lusignan or de Valence was a French nobleman and Knight, who became important in English politics due to his relationship to Henry III. He was heavily involved in the Second Barons' War, supporting the King and Prince Edward against the rebels led by Simon de Montfort. He took the name de Valence ("of Valence").
He was the fourth son of Isabella of Angoulême, widow of king John of England, and her second husband, Hugh X of Lusignan, Count of La Marche, and was thus a half-brother to Henry III of England, and uncle to Edward I. William was born at Valence, near Lusignan.
William was soon married to a great heiress, Joan de Munchensi or Munchensy (c. 1230 – after September 20, 1307), Lady of Swanscombe and Countess of Pembroke, the only surviving child of Warin de Munchensi, Lord of Swanscombe and Earl of Pembroke, and wife Joan Marshal, daughter and eventual co-heiress of William Marshal, 1st Earl of Pembroke and Isabel de Clare, 4th Countess of Pembroke suo jure.
William and Joan de Munchensi (described above) had the following children:
Isabel de Valence (d. October 5, 1305), married before 1280 John Hastings, 1st Baron Hastings (May 6, 1262 – February 10, 1313). Their grandson Lawrence later became earl of Pembroke. They had:
William Hastings (1282 – 1311)
John Hastings, 2nd Baron Hastings (September 29, 1286 – January 20, 1325), married to Juliane de Leybourne (d. 1367)
Sir Hugh Hastings of Sutton (d. 1347)
Joan de Valence, married to John Comyn (the "Red Comyn"), Lord of Badenoch (d. murdered, February 10, 1306), and had
Elizabeth Comyn (November 1, 1299 – November 20, 1372), married to Richard Talbot, Lord Talbot
John de Valence (d. January, 1277)
William de Valence (d. in the Battle of Llandeilo Fawr in Wales in June 16, 1282), created Seigneur de Montignac and Bellac
Aymer de Valence, 2nd Earl of Pembroke and Wexford in 1296 (c. 1270 – June 23, 1324), married firstly to Beatrice de Clermont and married secondly to Marie de Chatillon
Margaret de Valence
Agnes de Valence (b. about 1250)

William served in 1250 in the 7th crusades.